An ion trap can be used to perform mass spectrometric chemical analysis, in which gaseous ions are trapped and ejected according to their mass-to-charge (m/z) ratio. The ion trap can dynamically trap ions from a measurement sample using a dynamic electric field generated by one or more driving signals. The ions can be selectively ejected corresponding to their m/z ratio by changing the characteristics of the electric field. The mass and relative abundance of different ions and ion fragments can be measured by scanning the characteristics of the electric field.
A typical mass spectrometer comprises an ionization source to generate ions from a measurement sample, an ion trap to separate ions according to their mass (or more specifically, mass to charge ratio), and an ion detector to collect filtered/separated ions and measure their abundance.
Tandem mass spectrometry (also referred to as MS/MS, MS2, MSn, etc.) refers to a mass analysis method in which ions may be first formed and stored in an ion trap, and then an ion of particular mass (which may be a parent ion or a fragment ion of the parent) may be selected from among them by isolating the parent ion from all other ions. The ion of interest may then be further dissociated by collisions with neutral species or other means to generate fragment ions (daughter ions). The daughter ions may then be ejected from the ion trap and analyzed using mass spectrometry techniques. One or more daughter ions can be further isolated and dissociated, thereby forming a chain analyses.
To isolate an ion for purpose of tandem MS, an RF trapping field may be scanned or ramped up to eject ions except for those having an m/z ratio of the ion of interest. The RF trapping field voltage or other system parameters such as the pressure may be adjusted and the remaining ions may be dissociated. Finally, the RF trapping field voltage may then be scanned again to allow the system to analyze any daughter ions resulting from any subsequent fragmentation.
Another method is to employ a second fixed frequency signal (in addition to the RF trapping field signal) to the ion trap. The fixed frequency is at a secular frequency in which a particular ion is resonant. The ion excited at its resonant frequency may gain energy rapidly and be ejected from the trap. If the secular frequency of a particular ion of interest is known, an excitation signal may be constructed to isolate the ion of interest by including frequency components of all other ions in the ion trap but not the secular frequency of the ion of interest. In this way, all the other ions can be ejected at once, leaving only the ion of interest in the trap. It may be desirable to isolate at least one ion in the trap, in which several frequencies components may be “skipped.”
A typical method of constructing such an excitation signal is to perform stored waveform inverse Fourier transform (SWIFT), in which a time domain waveform corresponding to a desired frequency spectrum is calculated using inverse Fourier transform by a computer and downloaded to a signal generator of the ion trap. Because inverse Fourier transform is computationally complicated and time consuming, a typical SWIFT takes a relatively long time to finish, such as up to ten minutes. Therefore, it is desirable to develop ion trap systems and corresponding analyzing methods for performing tandem mass spectrometric analysis with improved speed, such as in real time.